Human lung ventilator system

ABSTRACT

A patient ventilator system provides a controllable flow and volume of a mixed inhalation gas to a patient, and receives exhaled gas from the patient. The ventilator system includes gas flow controllers that mix air and oxygen, and provide the mixture to the patient with a controllable inhalation cycle. The ventilator system monitors the resulting patient pressures and volume of gas exhaled by the patient, as necessary. Compressed air is provided to the gas controller in the event of an absence of an external gas supply by a compressor. A controller permits the user of the ventilator system to control ventilation mode, gas pressure, composition, and flow rate of the mixed gas with a single control knob.

This is a continuation, of application Ser. No. 07/535,191, filed Jun.7, 1990, now U.S. Pat. No. 5,237,987.

BACKGROUND OF THE INVENTION

This invention relates to apparatus for assisting or forcing a person tobreathe, and, more particularly, to a ventilator system that is flexiblein operation and easily controlled by a user.

In many cases the discomfort of critically ill persons can be eased, andtheir recovery hastened, by a proper program of breathing assistancesupplied by a device termed a "ventilator". In simplest terms, theventilator either forces pressurized gas into the lungs (e.g., apositive-pressure ventilator) or expands the chest cavity of the patientto draw gas into the lungs (e.g., a negative-pressure ventilator) undera selectable schedule of gas composition, pressure, and flow pattern.While negative-pressure ventilators enjoyed a degree of popularity inthe past, their use has been largely replaced by positive-pressureventilators, and the present invention relates to such positive-pressureventilators.

The ventilator typically includes a compressor that supplies pressurizedair, or the ventilator may operate from hospital pressurized air andoxygen lines. The gas is provided to the patient for inhalationaccording to a prescribed schedule, such as, for example, a specificpressure profile or a specific gas volume delivery profile with time.The inhalation gas flows to the patient and into the lungs. Manyventilators can be adjusted to either force breaths or respond only tothe patient's attempts to breathe and assist in that breathing, oroperate in some more complex pattern.

The exhaled gas that flows from the patient may also be controlled. Forexample, in some cases it has been found useful to maintain the exhaledgas under positive pressure, and the ventilator provides a positive endexpiratory pressure (PEEP) mechanism for that purpose. A conventionalPEEP mechanism restricts exhalation by closing off the path for exhaledgas flow when airway pressure drops below the pre-set PEEP level.

A primary consideration in the design of ventilators is safety, in termsof both avoiding adverse effects of apparatus failure and ensuring thatthe ventilator aids the patient's own efforts to breath. The ventilatormust cooperate with the efforts of the patient to breathe, and indeedthe ventilator must permit the patient to be "weaned" from fullventilator dependence to self sufficiency. Ideally, the ventilatorshould never work against the patient's own efforts. Instead, theventilator may provide aid for patient-induced breaths, may induceventilation without the assistance of the patient, or may accomplish acombination schedule of permitting or assisting the patient withself-triggered breaths and then ventilating without patient assistancebetween the patient's own breaths.

Respiratory therapy has developed into a complex field as more has beenlearned about the beneficial effects of proper ventilation in a varietyof circumstances. Doctors are trained to understand the requirements ofproper gas supply to the lungs, and to determine a proper schedule ofpatient ventilation that is preferred in particular types of cases. Forexample, the patient with emphysema normally requires quite a differentventilation schedule than the patient recovering from chest surgery.

Early ventilators used all-pneumatic systems for control of gas flow,gas blending, breath rates, patient breath assistance, and pressurecontrol. These systems provided little monitored data and few (or no)alarms and therefore relied heavily on the skill and diligence of theoperator to establish and maintain ventilation parameters. Latergenerations of ventilators contained electronic circuits which providedmore precise control of timing parameters such as breathing rates andinspiratory time, and had pressure and flow measuring devices to providedisplays of monitored data and to facilitate alarm activations ifpatient airway pressures, breath-to-breath gas volumes, or frequency ofbreaths were outside of user-established limits.

The current generation of ventilators use microprocessors to controlmost of the parameters of ventilation and contain pressure and flowmeasurement transducers which provide electrical data (viaanalog-to-digital converters) to the microprocessors for display ofmonitored parameters and for alarm activations. These microprocessorbased ventilators, as compared to previous generations, may haveimproved flow and pressure control accuracy, may display data in graphicform and present additional data based upon mathematical manipulationsof pressure and flow data, and may offer improved safety features. Amain advantage of microprocessor based ventilators is the ability to addnew features by changing only the memory integrated circuits (usuallyEPROMs) containing the software programs.

One major disadvantage of some current designs of microprocessor basedventilators is the complexity of the user interface. Typically,ventilator parameters are either input by control knobs, with one knobfor each parameter setting as in the prior electro-mechanicalventilators, or by keyboard entry. Parameter settings are typicallydisplayed on seven-segment type displays, either continuously, uponselection, or sequentially in ticker-tape fashion. Likewise, monitoreddata is displayed on seven-segment type displays. Because of spacelimitations on the control panel, not all monitored data can bedisplayed at the same time, and the desired data must be selected fordisplay by selection switches.

As new features and new ventilating modes are added, the complexity ofoperation increases because the existing controls and display areas mustbe burdened with the requirement of facilitating input and display ofthe new features. The microprocessor controlled ventilator also tends tobe more costly than previous generations because of the need fordesigning unique pneumatic control and monitoring devices which arecontrollable by the microprocessor as well as the need for anon-volatile memory for storage of ventilation parameter and alarmthreshold settings. The exhaled gas measurement transducer as an examplehas traditionally been difficult to design and very expensive to buildbecause of the requirements of accurate flow measurement with very smallpressure drops.

Another problem with microprocessor based systems is theirsusceptibility to AC power line noise. Voltage spikes and other forms ofelectrical noise can pass through conventional power supplies andinterfere with the microprocessor's ability to access the program codefrom ROM memory and in reading and writing of data to RAM memory.Momentary drop-outs of power line voltage can cause the ventilator powersupply's regulated voltages to drop below the operating limits of theintegrated circuits, with unpredictable consequences. Power supplies formicroprocessor based ventilators must therefore be designed to be immuneto all forms of power line noise. Such immunity is normally accomplishedwith elaborate line voltage spike suppressors and EMI filters.

Conventional ventilators are without battery back-up power supplies andtherefore must be designed to shut down in a safe manner with alarmsactivated if the line voltage momentarily drops below acceptable limits.These requirements are difficult to achieve, and it is nearly impossibleto verify that the system is immune to all forms of electrical noise andall durations of power line drop-outs. Hospital power is always backedup by a motor-generator which starts up when the power line drops out.The switch-over in power sources which occurs during a power lineblackout or brownout can cause momentary voltage dropouts, voltagespikes, power line frequency shifts, etc., which in turn can causemicroprocessor (and other electronic) based systems to fail or shutdown. In addition, occasionally the back-up power systems themselvesfail, causing all electronic systems in the hospital which are notbattery backed to shut off, thereby causing a hazardous situation forthe patients.

Thus, there is a continuing need for a ventilator system that is easierfor hospital personnel to use, less expensive, and more tolerant offault situations such as powerline problems. The ventilator must be areadily controlled, convenient apparatus that supplies the requiredventilation conditions with minimal chances of error, either in settingthe ventilation conditions or in meeting the set schedule. The presentinvention fulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides a ventilator system that is exceptionallyconvenient to use, and is precisely controllable to permit a selectedventilation schedule to be provided to the patient. Flexibility andprecision are included in the operation of the ventilator system, withcareful engineering to reduce the costs of the apparatus as much aspossible consistent with the overriding concern for patient safety. Theapparatus provides a compact unit that is well suited for hospital orother patient-care environments.

In accordance with the invention, a ventilator system for use inassisting a patient to breathe comprises gas control means for mixingand regulating the gas that is to flow to a patient and receiving thegas that flows from the patient; and controller means for controllingthe gas control means, the controller means including a single knob thatcontrols ventilation mode, gas pressure, gas composition, and flow rateof the gas that flows to the patient.

In another aspect of the invention, a ventilator system for use inassisting a patient to breathe comprises gas control means for mixingand regulating the gas that is to flow to a patient and receiving thegas that flows from the patient, the gas control means including flowcontrol means for mixing together controllable amounts of two gases toform a mixed gas that flows to the patient, the flow control meansincluding a custom-calibrated rotary flow control valve for each of thetwo gases; and controller means for controlling the gas control means.

In a preferred approach, the inhalation flow schedule of gas pressure,flow, and composition as a function of time is determined by apressurization/flow controller that receives pressurized gas ofessentially constant pressure, either from a compressor or from anexternal source. Two (or more) gases, typically air and oxygen, aremixed together to form a mixed gas composition that is supplied to thepatient. The amount of each gas and its flow rate as a function of timeare established by an open loop gas control system using controllablestepper-motor-driven rotary flow control valves that are individuallycalibrated and periodically zeroed during operation. This system has theadvantage that it is simpler than closed loop systems. The flow rate ofthe mixed gas is determined by measuring the pressure drop across anadjustable gas orifice, which is more readily and accuratelyaccomplished than a direct flow rate measurement.

The inhalation gas mixture then is conducted to the patient, firstpassing through a conventional bacteria filter and humidifier. Anebulizer subsystem supplies medication to the gas mixture upon demand.

Exhaled gas from the patient passes through a tube to a filter. A watertrap serves as a drain and reservoir for moisture condensed in thetubing. Alternatively, the tubing may be heated to prevent condensation.The pressure of the exhaled gas is controllable with a positive endexpiratory pressure (PEEP) subsystem that applies a selectable pressureprofile to the exhaled gas.

In some situations a supply of filtered compressed air is not available,and the ventilator system includes a compressor that provides such asupply. Since the ventilator system is normally operated in the room ofa sick or injured person, an important virtue is quiet operation. Ashortcoming of many prior ventilator compressors is that they generateexcessive noise because they operate continuously at maximum output flowrate and pressure. More over, continuous-operation compressors must besized for maximum gas flow rates. The present compressor motor runsconstantly, with valves to alternately fill an accumulator and thenidle, pumping against no load. The accumulator in turn supplies gas tothe pressurization/flow controller. The compressor may therefore beprovided in a smaller, quieter size and structure than with anon-cycling compressor.

Conventional ventilator controllers utilize an array of separate controlswitches and knobs to set the gas flow schedule, flow rates, pressures,PEEP schedule, and other control parameters. The myriad of controls canbe confusing to even a well-trained operator, particularly in thecircumstances of a critical patient situation. The present controllerutilizes microcomputer-based window and menu manipulation software topermit all significant control functions to be determined with a singlecontrol knob. The control parameters, status alarms, and monitored dateare displayed on a clearly delineated cathode ray tube (CRT) monitorscreen so that the operator can see the current status and how thatstatus is to be affected by any particular adjustment.

The present invention therefore provides an important advance in the artof ventilator technology, both in ventilator operation and in ventilatorcontrol. The unit is simple to operate with a single knob that entersall command functions. On-screen information presents all command, data,and alarm information, and even displays prompt and assistanceinformation to the user. Low-cost metering valves are precise, yetreadily calibrated and controlled. A plurality of microprocessors areused to achieve wide flexibility in system functions. The electronicsoperates from a battery that is continuously charged when line voltageis available, and the battery filters noise from the power signal sothat it does not reach the electronics. The improved apparatus iseconomically competitive with conventional apparatus, and has improvedsafety, ease of use, and reduced noise. Other features of the presentinvention will be apparent from the following detailed description ofthe preferred embodiment, taken in conjunction with the accompanyingdrawings, which illustrate, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the overall arrangement of the ventilatorsystem;

FIG. 2 is a block diagram of the pressurization/flow controllersubsystem;

FIG. 3 is a schematic perspective cutaway view of a rotary gas flowcontrol valve;

FIG. 4 is a block diagram of the nebulizer subsystem;

FIG. 5 is a block diagram of the exhalation subsystem;

FIG. 6 is a block diagram of the PEEP subsystem;

FIG. 7 is a block diagram of the compressor subsystem;

FIG. 8 is a block diagram of the controller subsystem;

FIG. 9 is a state machine diagram for the ventilator system;

FIG. 10 is a state diagram for the graphics interface; and

FIG. 11 is a block diagram for the power supply of the ventilator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment resides in a ventilator system used to assist apatient to breathe. The structure of the ventilator system as a whole isillustrated in FIG. 1, and FIGS. 2-11 illustrate the details of varioussubsystems.

In accordance with a preferred embodiment of the invention, a ventilatorsystem for use in assisting a patient to breathe comprises gas controlmeans for mixing and regulating the gas that is to flow to a patient andreceiving the gas that flows from the patient, the gas control meansincluding flow control means for mixing together controllable amounts oftwo gases to form a mixed gas that flows to the patient, the flowcontrol means including a custom-calibrated rotary flow control valvefor each of the two gases, an inhalation flow sensor through which themixed gas flows, the inhalation flow sensor including a screen throughwhich the mixed gas flows and a pressure sensor that senses the pressuredrop across the screen, whereby the mixed gas flow rate is determinedwith a preexisting calibration relationship, a nebulizer that injects acontrollable flow of a third component into the mixed gas stream,ventilation tube means extending to the patient for conducting the mixedgas to the patient and for receiving exhaled gas from the patient, theventilation tube means including a filter for the mixed gas, ahumidifier for the mixed gas, a water trap for the exhaled gas, and afilter for the exhaled gas, and positive end expiration pressure meansfor controllably pressurizing the exhaled gas; compressor means forsupplying compressed gas to the gas control means, in the event that anexternal supply of compressed gas is not available, the compressor meansincluding a compressor that operates continuously against a compressoraccumulator and is sized to meet average rather than peak gas flowvolume; and controller means for controlling the gas control means andthe compressor means, the controller means permitting the setting ofventilation mode, gas pressure, gas composition, and flow rate of themixed gas with a single control knob.

Referring to FIG. 1, a ventilator system 20 includes a gas controllersubsystem 22 that supplies a controlled flow of inhaled gas to thepatient and also controls the pressure of the exhaled gas, a compressorsubsystem 24 that supplies pressurized gas to the gas controllersubsystem 22, a gas conditioner subsystem 25 that alters the compositionof the gas flows by filtering, humidifying, dehumidifying, and asotherwise required, and a controller subsystem 28 that controls theoperation of the other components.

The gas control subsystem 22 includes a pressurization/flow controller30 that is supplied with pressurized oxygen from an external pressurizedoxygen source 32, and with pressurized air from an external pressurizedair source 34 or the compressor subsystem 24. The oxygen and airsupplied to the gas control subsystem 22 preferably have a pressure offrom about 30 to about 90 psig (pounds per square inch, gauge). The gascontrol subsystem 22 regulates the pressure of the oxygen and air, andthen mixes them according to the commands received from the controllersubsystem 28, in a manner to be described in detail subsequently. Theresulting mixed gas flow 35 has a particular composition, flow rate, andpressure profile precisely determined according to the controllersubsystem commands.

The mixed gas flow 36 passes to the gas conditioner subsystem 26 througha manifold 31, prior to reaching the patient for inhalation. The gasconditioner subsystem 26 alters the composition of the gas stream, notwith respect to the oxygen and air content, but instead with respect toother components that are desirably or undesirably present. A filter 38removes microscopic particles such as bacteria from the gas flow 36 thatmight adversely affect the patient. A humidifier 40 adds moisture to thegas flow 36, as might be required for a particular patient, andparticularly when the atmospheric humidity is low. A nebulizer injector42 and its associated nebulizer 48 add medication, such as for example adecongestant, to the gas flow 36.

A stream of exhaled gas 43 from the patient is also processed throughthe gas conditioner subsystem 26. The exhaled gas is passed through awater trap 44 to remove excess moisture and through an exhalationsubsystem 46, to be described in more detail subsequently, that includesa filter for the exhaled gas and a gas flow monitor.

The filter 38, humidifier 40, nebulizer injector 42, trap 44, and tubingthat forms the Y leading to and from the patient are all commerciallyavailable products, and are well known in the art.

A PEEP subsystem 50, which is part of the gas controller subsystem 22,controls the pressure in the airway during expiration. The PEEP(positive end expiratory pressure) subsystem 50 prevents the pressure inan airway 52 from falling below some preselected minimum value duringexhalation, so that a positive pressure is retained in the lungs at theconclusion of the expiration of air. A doctor may choose such therapyfor a variety of reasons, such as to prevent collapse of the alveoli ofthe lungs during exhalation.

Pressure in the airway 52 is measured by a proximal air pressure sensor54 at a point near the Y of the tube extending to the patient.

FIG. 2 illustrates the pressurization/flow controller 30 in greaterdetail. Oxygen is provided to the system through the oxygen source 32,and air is provided either from the air source 34 or a line 60 from thecompressor subsystem 24. The pressure in the oxygen side of the gassupply is sensed by an oxygen pressure sensor 62, and the pressure inthe air side of the gas supply is sensed by an air pressure sensor 64.Check valves 66 prevent gas flow in the directions that, but for thecheck valves 66, would result in possible contamination of the gassupplies by the other gas.

The oxygen passes through a regulator 68 wherein its pressure iscontrolled to the range of about 61/2 to 71/2 pounds per square inch,gauge (psig) and then through a filter 70 having a filter element thatremoves particles of less than about 0.3 micrometers. Similarly, the airpasses through a regulator 72 and filter 74 that operate identically.

The flow rate of oxygen is established by a rotary valve 76, and theflow rate of air is established by a rotary valve 78. These valvesprovide the basic control function for the gas flows to the patient, andtheir construction will be discussed in more detail subsequently. Thegas outflows from the valves 76 and 78 are mixed and provided to anoutflow line 81.

The pressure drop across the oxygen rotary valve 76 is measured by anoxygen pressure drop transducer 82, and the pressure drop across the airrotary valve 78 is measured by an air pressure drop transducer 83. Thepressure drop in each case is a function of upstream pressure and flowrate through the valve. Pressure drops outside of established limitsindicate a malfunction of the valves 76 and 78, if such should occur.

In the event of a malfunction of one of the valves 76 or 78, a valve 88is operated to permit a direct flow of gas into a stop valve 80 andpressurize a diaphragm 89 therein. The diaphragm flexes to the right inthe view of FIG. 2, closing the outflow line 81. There is no gas flowthrough the outflow line 81 to the patient. This safety mechanismprevents the full force of the inlet gas being applied to the patient'slungs.

The mixed gas in the line 81 is filtered in a filter 90 having a filterthat removes particles of less than about 0.3 micrometer size.

The flow rate of the mixed gas is sensed in a flow rate sensor 92. Thegas flows through a sensor body 94 having a wire mesh screen 96 therein,through which the gas must flow. The pressure of the gas drops slightlyas it passes through the screen 96, the pressure drop being a functionof the flow rate of the gas. That pressure drop across the screen 96 issensed by a pressure transducer 98, whose reading is thereforeindicative of the flow rate of the mixed gas. A valve 104 is openedperiodically to connect the two sides of the transducer 98, and isolateit from the high pressure side of the screen 96, to permit thetransducer 98 to be zeroed. The gas flowing from the flow rate sensor 92enters the manifold 31 through which it flows to the gas conditionersubsystem 26. The pressure in the manifold 31 is sensed by a pressuretransducer 106.

The manifold 31 is provided with two further safety switches. Asubambient relief valve 108 opens if the pressure within the manifold 31becomes less than ambient air pressure for any reason, and permitsoutside air to flow into the manifold 31. A safety valve 110 operated bya safety pilot valve 216 permits outside air to flow into the manifold31 under three conditions. The valve 110 is operated if the system isoff or there is a malfunction that cuts off the pressurized gas orelectricity. Second, the valve 110 is operated if an overpressure in themanifold 31 is sensed. Third, the valve 110 acts as a pop-off valve whenthe pressure in the airway exceed a specified holding pressure. Theseconditions are activated by malfunction states that are not expected innormal operation, but must be accommodated should they occur. In theabsence of such relief valves, the patient might be deprived of any gasor air for breathing, or the patient's lungs might be overpressurized.Provision of unpressurized air is preferable to such deprivation oroverpressurization. Each condition is accompanied by a signal to theoperator of the ventilator 20 that a malfunction is occurring, so thatthe operator may investigate and correct it.

A key feature of the pressurization/flow controller 30 is the rotaryvalves 76 and 78, that control the flow of oxygen and air, respectively,to the patient. Many gas flow controllers use feedback systems, whichare relatively expensive. The valves 76 and 78 are not feedback orclosed loop valves, but instead are individually calibrated andperiodically zeroed to ensure their correct operation. The two valves 76and 78 operate substantially identically, and therefore only the oxygenvalve 76 will be described in detail.

As depicted in FIG. 3, the valve 76 includes a stationary plate 120having a triangular opening 122 therethrough. A movable disk 124 havinga cam-shaped outer periphery 126 lies adjacent to and in contact withthe stationary plate 120, to partially cover the opening 122. Themovable plate 124 rotates on a shaft 128 in the center thereof, drivenby a stepper motor 130. The stepper motor 130 of the preferredembodiment has about 0.45 degrees of rotation per step. The position ofthe disk 124 is determined by counting steps from the zeroed position.With each step, the coverage of the opening 122 changes slightly, sothat either more or less gas is permitted to pass through the opening122. A gas inlet line 134 to the valve 76 is on one side of thestationary plate 120, and a gas outlet line 136 from the valve 76 is onthe other side of the stationary plate 120. The gas inlet line 134 ispressurized, and that pressure forces the gas through the opening 122with a flow rate that depends upon the coverage of the opening 122 bythe movable disk 124.

This type of valve 76 is highly reliable in its operation, amenable tocomputer control due to its digital stepping nature, fast acting, andhas a high resolution. Because of the low pressures and flow rates ofthe gas and the large number of control steps, even normal variations indimensions or operation of the valve 76, due to manufacturingtolerances, can lead to slight differences in the performance of eachvalve.

A calibration and rezeroing procedure is used to obviate these normalvariations in operation of each valve. When each valve is manufactured,it is individually calibrated to establish a quantitative relationshipbetween gas flow rate at a specified pressure (typically about 7 psig)and degree of rotation (as measured by counting the number of motorsteps) of the movable disk 124 from a fully closed zero point. Thiscalibration is prepared in the form of normalized values that can beused for any pressure drop, arranged in a table, and stored in thecontroller 28 in computer memory. When the ventilator 20 is inoperation, the movable disk 124 is rotated to the fully closed positionon a periodic basis (every eight breaths in the preferred embodiment) toestablish the number of the motor step that corresponds to the fullyclosed position. The updated zero point is then used in conjunction withthe calibration to calculate the rotary position required to provide aselected gas flow.

The nebulizer 48 is illustrated in FIG. 4. The nebulizer is a device forinjecting controlled amounts of medication, such as for example adecongestant, directly into the respiratory tract of the patient. Anebulizer pump 140 receives a flow of the same mixture of oxygen and airbreathed by the patient from the manifold 31, and pressurizes it toabout 8-10 psi pressure. The gas is pumped through a filter 141 thatremoves particles smaller than about 0.3 micrometers, and to thenebulizer injector 42, where medication is mixed into the gas stream. Inthe event that a subambient condition occurs within the system, the pump140 receives outside air through the valve 108 and continues to pump theair to the nebulizer injector. A nebulizer safety valve 144 has twostates, one where the input side of the pump 140 is connected to theoutput side to prevent pressurized gas flow to the nebulizer, and theother where the gas flow is through the pump 140. The valve 144 normallyoperates in the second state so that the pump 140 forces gas to thenebulizer injector 42, allowing nebulization at low flow rates whilestill maintaining accurate blending. The valve 144 may be periodicallyswitched to the first state to prevent too high a flow rate to thenebulizer injector 42, or to prevent drawing a vacuum on the manifold31.

The exhalation subsystem 46 is illustrated in FIG. 5. The exhaled gasfrom the patient first passes through the water trap 44 to drain thetubing leading from the patient. The gas is then filtered with a heatedbacteria filter 150, which preferably removes particles of less thanabout 0.3 micrometer, to prevent bacteria exhaled by the patient frombeing introduced into the ambient air and to prevent contamination ofthe exhalation subsystem 46. Such bacteria filters are known in the art.

The filtered gas passes through an exhalation valve 152 which carriesthe gas from the filter 150. When a diaphragm 154 in the valve 152 isfully flexed to the right in the view of FIG. 5, the inlet line 156 isclosed by the diaphragm 154 and no gas can flow therethrough. As thediaphragm 154 flexes to the left in the view of FIG. 5, an inlet line156 becomes progressively more opened so that the gas therein can flowmore freely. By moving the diaphragm 154 to the left or right, theresistance to the flow of exhaled gas in the line 156 is controlled.Thus, for example, if the therapist wishes to maintain the patient'slungs inflated at the end of a breath, the diaphragm 154 and the valve152 are closed at the end of the breath so that the last portion of theexhaled gas is retained in the lungs of the patient.

The flexure of the diaphragm 154 is controlled by pressurizing the backside of the diaphragm 154 through a diaphragm control line 158. Theopening against which the diaphragm 154 seats has a preciselydimensioned diameter to produce a well-defined force of area timespressure against the right-hand side of the diaphragm. The force againstthe left-hand side of the diaphragm 154 is determined by its area timesthe pressure against the left-hand side. To maintain a forceequilibrium, the pressure against the left-hand side is varied throughthe control line 158 by the PEEP subsystem, in the manner to bediscussed subsequently.

The air passing through the valve 152 next flows through an exhalationflow sensor 160. The sensor 160 functions in the same manner as theinhalation flow rate sensor 92. The exhaled gas passes through a acre en162, which causes a pressure drop. The pressure in the sensor 160 ismeasured upstream of the screen 162 and downstream of the screen 162 bya differential pressure transducer 164, and the pressure differencerelated to the gas flow rate. A zeroing valve 166 is periodicallyoperated to connect the high pressure side and the low pressure side ofthe transducer 164, so that it may be zeroed. In normal operation, thevalve 166 connects the high pressure side of the transducer 164 to thehigh pressure side of the screen 162. The gas leaving the exhalationflow sensor 160 is released to the atmosphere.

The PEEP subsystem 50, illustrated in FIG. 6, controls the pressure onthe back side of the diaphragm 154 of the exhalation valve 152. The PEEPsubsystem 50 has two important functions. First, during inspiration ofthe patient, the exhalation valve 152 must be held firmly closed, sothat the air flow from the pressurization/flow controller 30 flows intothe patient and not out through the exhalation system 46. Second, duringexpiration of air by the patient, the valve 152 is opened during part ofthe expiration period but may be closed or partially closed toward theend of the expiration period to maintain pressure in the patient'slungs. Because of the pressure cycling and accumulation requirements ofthe PEEP subsystem, the first and second functions are performed bydifferent but interrelated parts of the PEEP subsystem.

Pressurized gas is supplied to the PEEP subsystem 50 through a line 200from the pressurization/flow controller 30. The pressure of the gas isregulated by a regulator 202 to a pressure of about 5-7 psig, andfiltered by a filter 204 that removes particles larger in size thanabout 0.3 micrometers. The filtered, regulated gas enters a manifold 206from which various components of the subsystem 50 are supplied.

The first subsystem function, closure of the exhalation valve 152 duringinspiration of gas into the patient, is accomplished by connecting thebackside of the diaphragm 154 to an IMV regulator 208 through anexhalation pi lot valve 210. The diaphragm 154 is held closed, so thatair flows from the pressurization/flow control subsystem 30 into thepatient. A safety valve 212 is placed between the exhalation pilot valve210 and the exhalation valve 152, so that, if there is an equipmentmalfunction such as an equipment-caused overpressure, the backside ofthe diaphragm 154 can be vented to atmosphere to maintain the exhalationvalve 152 in an opened state.

Other situations involving abnormally high pressures can also occur, aswhen the patient coughs. In that event, the valve 212 operates to ventto atmospheric pressure, thereby reducing the pressure on the back sideof the diaphragm 154 to atmospheric and opening the exhalation valve152. The abnormally high pressure created by the patient is therebyvented, even though the system is inspirating at the time of the cough.

During exhalation of gas by the patient, the valve 152 is controlled bythe PEEP subsystem 50 to maintain a therapist-selected minimum baselinepressure in the airway. Again, the control is achieved by applying abackside pressure to the diaphragm 154 of the exhalation valve 152, notby directly forcing gas into the airway. The backside pressurizing gasis supplied by a PEEP regulator 218 through the pilot valve 210, whichswitches between regulators 208 and 218 during inspiration andexhalation portions of the breath, respectively.

When a PEEP setting is selected by the therapist, a PEEP pressurecontrol valve 220 is pulsed at a preselected duty cycle to produce anappropriate pressure at the inlet of the dome diaphragm 222 of the PEEPregulator 218. The connection to the regulating diaphragm 222 occursthrough an accumulator 224 and in conjunction with flow restrictors 226and 227 to reduce the magnitude of the pulsations that could otherwisebe transmitted to the diaphragm 154.

The PEEP pressure control valve 220 does not produce sufficiently highflow volumes to operate the large diaphragm 154 at the speeds typicallyrequired, and the PEEP regulator 218 acts as an amplifier of thepressure signal produced by the valve 220. The dome pressure 222 of theregulator 218 controls the pressure in the output line 228 of theregulator 218, and thence the pressure applied to the backside of thediaphragm 154. The pressure on the diaphragm 154, and thence thebaseline pressure in the airway 52 between breaths, is therebycontrolled by changing the duty cycle of the PEEP pressure control valve220.

The exhalation valve 152 could be operated with the arrangement justdescribed, but would not produce the most preferred performance, for thefollowing reason. The pressure output of the regulator 218 variesnonlinearly with the gas flow rate to the dome diaphragm 222, andtherefore the pressure applied to the diaphragm 154 of the exhalationvalve 152 varies nonlinearly with duty cycle of the valve 220. Amodification to the system causes the pressure applied to the exhalationvalve 152 to vary more linearly with the duty cycle of the valve 220.This approach is open loop for safety, but with minor servo action foraccuracy.

The output flow of the valve 220 is shaped to achieve more nearly linearperformance by providing a variable accumulator 230 upstream of the PEEPpressure control valve 220. The variable accumulator 230 is a chamberwhose volume is controllably varied, depending upon the pressurerequirements on the output of the valve 220. A flow restrictor 232upstream of the valve 220 and the variable accumulator 230 limits theflow through the valve 220 when the valve is open, and determines thetime required to refill the accumulator 230 when the valve 220 isclosed. When the valve 220 opens, the flow through the valve 220 isinitially high as the variable accumulator 230 exhausts quickly, thenreduces to a steady flow dictated by the inlet restrictor 226 and thepressure setting of the regulator 202. When the valve 220 closes, thevariable accumulator 230 begins charging for the next cycle.

For "low" duty cycles where the "on" time of the valve 220 is short(i.e., the valve 220 is open for a small fraction of the time), there isa high flow rate through the valve 220 because of the rapid discharge ofthe variable accumulator 230. An increase in the "on" time significantlyincreases the average flow rate. For "high" duty cycles, increasing the"on" time by a small amount adds only marginally to the average flowrate, because the variable accumulator is nearly exhausted after a longperiod of outflow. A high duty cycle results in incomplete filling ofthe accumulator 230, further decreasing the average flow rate.Increasing the volume of the variable accumulator 230 magnifies thiseffect, and is used to boost flow at low duty cycles. The volume of theaccumulator 230 has little effect on the steady flow reached after long"on" times, because this flow is a function of the pressure upstream ofthe inlet restrictor 232. The regulator 202 is adjusted to determine theupstream pressure in the manifold 206 with the valve 220 at a 100percent duty cycle, fixing the high end of the scale. The variableaccumulator 230 is then adjusted to calibrate the circuit at low dutycycles without affecting the high end adjustment.

An accumulator bag 234 supplements the output air flow of the regulator218 after exhalation, thereby rapid repressurizing the backside of thediaphragm 154 to the pressure level required by the PEEP setting. Thisrapid repressurization prevents undershooting of the PEEP pressureimmediately after exhalation. A flow restrictor 236 causes slightoverfilling of the accumulator bag 234 during inspiration to furtherinsure that no PEEP pressure undershooting can occur when the backsideof the diaphragm 154 is refilled.

The compressor subsystem 24 is illustrated in FIG. 7. A compressor 250draws in air through a filter 252, and compresses the air. Thecompressed air is cooled by a heat exchanger 254 to condense moistureand then stored in an accumulator 256. When there is a demand from thepressurization/flow controller 30, compressed air flows out of theaccumulator 256 through the line 60. The compressed air passes through awater trap 258 to remove any moisture that may have condensed.

The maximum pressure of the accumulator 256 is established by acontrollable compressor unloading valve 260 that communicates with theoutput of the compressor 250. The compressor unloading valve 260 has asilencer thereon to reduce the noise level in the ambient environment.The release pressure sensed by a pressure transducer 266 is used by thecontroller 28 to control a compressor unloading pilot valve 262, whichin turn controls the compressor unloading valve 260.

To ensure that the system cannot overpressurize, a manual pressurerelief valve 264 communicates with the output of the compressor 250, sothat the pressurized air in the accumulator 256 cannot exceed thepressure set on the relief valve 264. In a preferred embodiment, thepressure relief valve 260 is set to about 40 psig. A pressure sensor 266measures the pressure downstream of the accumulator 256.

The accumulator 256 and compressor 250 are together sized so that theamount of air available is sufficient to meet demand needs for theventilation of a patient, with the compressor 250 operatingcontinuously. The inclusion of the accumulator in the system reduces themaximum flow rate of air that must be supplied by the compressor to meetpatient needs during inhalation. The compressor may therefore be madesmaller and quieter than would be the case in the absence of theaccumulator. In a preferred embodiment, the usable volume of theaccumulator 256 is about 12 liters, and the compressor 250 is a ThomasIndustries 2619 Series, rated at 2.95 standard cubic feet per minute ofair flow at 15 psig (pounds per square inch, gauge) or greaterpressures. This compressor has a noise level of 60 decibels (db), whilea compressor sized to meet maximum load would have a considerablygreater noise level.

The controller subsystem 28 is illustrated in FIG. 8. In the preferredapproach, the controller 28 includes four microprocessor that monitorand/or control various aspects of the individual subsystems, and a mainmicroprocessor that integrates the operations of the othermicroprocessors. This approach permits the most efficient use of theelectronic components, and avoids unnecessarily complex interactivecomputer programming that might be necessary if all functions werepacked into a single microprocessor. Their programming is provided byEPROMs (Erasable Programmable Read Only Memories) that can be externallyprogrammed and duplicated, and then inserted into the microprocessorboards.

An A/D (analog to digital) microprocessor 280 receives the analogsignals from the various pressure transducers described previously, suchas transducers 54, 82, 83, 98, 106, and 164, and any others that mightsubsequently be added to the system. These signals are digitized, in thepreferred embodiment into 8 bit words, and conveyed to a shared RAM bus282.

A stepper motor driver microprocessor 284 controls the stepper motors 76and 78, according to the principles set forth previously herein. A Valveand Alarm Driver Board 286 controls the various open/close type solenoidvalves described previously, such as the valves 88, 104, 144, 166, 210,212, 216, 220, and 263, the nebulizer pump 140, and displays and alarmsthat convey information to the user of the ventilator 20.

A graphics microprocessor 288 is programmed to handle most of theoperator input and operator display functions, according to a process tobe described subsequently.

A main microprocessor 290 includes a battery-backed, shared RAM (randomaccess memory) that is accessible to all of the other microprocessors,and performs integration functions according to the procedures discussedpreviously.

The various microprocessors operate according to instruction sets, andusing data, provided in EPROMs. The processes can be represented bystate diagrams, from which detail code is written by those knowledgeablein microprocessor programming. FIG. 9 is a state machine diagram for theventilator control functions, and FIG. 10 is a state diagram for theuser interface. States are represented by "balloons", and arrowsrepresent transitions between states permitted by the microprocessorprogramming of the presently preferred embodiment. An important virtueof the present microprocessor-controlled ventilator, thestate-programming approach, and the use of programming stored on EPROMsis that the states and the transitions can be readily modified. Forexample, if a new philosophy of respiratory therapy is developed or if atherapist requires some new or unusual procedure for a particularpatient or condition, the existing states and transitions can bemodified, the programming modified and burned into an EPROM, and the newcapability supplied to all or some of the ventilators in use simply byreplacing the appropriate EPROM.

The various ventilation functions can be represented as the five statesdepicted in FIG. 9. The appropriate state, or sequence of states, isselected responsive to the profile selected by the therapist. A no-flowexpiration state 330 is that prior to or between breaths. A demand-flowexpiration state 332 is selected if the ventilator is operated to followand assist fully spontaneous patient breathing. That is, the therapistmay determine that the patient requires assistance in spontaneousbreathing, and the ventilator follows the breathing of the patient toprovide a particular mix of gases, a higher inspiration pressure, orpositive end expiratory pressure, for example. At the opposite end ofthe spectrum is a controlled inspiration state 334, where the ventilatorsupplies a breath without any assistance or effort by the patient. Thetherapist may select ventilator operation solely in the demand-flowexpiration state 332, solely in the controlled inspiration state 334, ora combination of the two. In the latter case, known as synchronousintermittent mandatory ventilation (SIMV), the operation of theventilator may be viewed as a sequential transition from state 330 tostate 332 for a patient breath, then a transition to state 334 for amachine inspiration, and finally a transition back to state 330 untileither the patient takes another spontaneous breath (state 332) or theventilator determines that another machine-driven inspiration isrequired (state 334). In each of the states 332 and 334, the operationof the machine such as gas mixture, pressures, volumes, times,nebulization, etc. is fully controllable by the therapist in the mannerto be discussed in relation to FIG. 10.

A Positive-Pressure-Support state 336 permits the pressure in the lungsof the patient to be raised when the patient draws a breath. Aninspiration hold state 338 is reached from the controlled inspirationstate 334, and provides the capability for the gas introduced during amachine breath to be held inside the patient for a period of time, afterwhich the breath is released and the system returns to the no-flowexpiration state 330.

The presently preferred microprocessor programming of these states andtransitions is presented in Appendix A, which is attached hereto andmade a part hereof.

The present ventilator therefore is highly versatile, in that it offersa wide assortment of options and sequences from which the therapist maychoose precisely the correct combination of respiratory therapy for asituation. This versatility, however, creates the need for a readilyunderstood and applied selection and monitoring approach, so that thetherapist can readily select the desired combination of states andconditions for each state and then monitor the functioning of theventilation activity.

To implement the control and monitoring functions, the present apparatususes a single cathode ray tube (CRT) to display all visual informationto the therapist, and as the vehicle for changing control parameters ofthe ventilator. In the preferred approach, most display information ispresented on one screen of information termed the "Display" screen, andcontrol parameter information is displayed and made changeable onanother screen of information, termed the "Control" screen. That is, theuser of the system can switch the single screen between two differentpresentations, one of which presents patient and system operating data,and the other of which permits the changing of system operatingparameters.

When the Control screen is viewed, all control functions to beaccomplished with one control knob 296 and one "enter" switch 298, aprocedure implemented with well-known "mouse"-type technology.Optionally but preferably, two additional switches, one to changescreens and the other to request help, are provided.

The relative simplicity of this approach is to be contrasted with thecomplexity of prior approaches, wherein information is displayed on amyriad of different meters and screens, and a variety of differentcontrols must be manipulated to control the system. It is not easy tomonitor patient functions and make control changes in the relative quietof a controlled situation using the prior approach, but in the noise andhaste of a hospital emergency setting there is a much greater chance oferror than when the present approach is used. Moreover, themicroprocessor/EPROM technique of the present invention, when coupledwith the concentration of display and control functions on a singlescreen, permits reprogramming and custom tailoring ventilator systems asneeded.

FIG. 10 is a state diagram of the user interface for controlling theventilator. This state diagram is interpreted by recognizing that theinterface is in one of the states illustrated by the "bubbles", and thenconsidering the options available. For example, when the interface is inthe "Select Setting" state 300, the user may select any of severaloptions, indicated by arrows extending out of the state 300. The usermay manipulate the control knob 296 to select a function to becontrolled, such as, for example, tidal volume, peak pressure, etc.,whose current values are displayed. If the user then presses the enterswitch 298, the state of the interface changes to an "Adjust Setting"state 302, and the selected quantity may be varied by turning thecontrol knob 296 until the desired value is reached. Pressing the enterswitch 298 again returns the interface to the select setting state 300,and establishes the manipulated setting in the microprocessor throughthe shared RAM.

The only other options available to the user in the Select Setting state300 are to press the screen change switch or the help switch, whereprovided. The screen change switch causes a CRT 58 to exhibit theDisplay screen, state 304, rather than the Control Screen. The helpswitch causes a listing of common problems or questions, and theirsolutions or answers, to be displayed, state 306. By combining thesefour functions (control knob, enter switch, help switch, and screenchange switch) in various ways, the user can accomplish the greatmajority of control and display operations required to use the system.

A second set of help information is available from the display state 304by pressing the help switch. The Select Help Alarm state 308 is entered,wherein the user can select how alarms are to be displayed, state 310.

Finally, from the help state 306, the user can select the date/clock tobe changed, state 312, by pressing the enter switch twice in one second.By pressing the enter switch again, the clock and date can be changedwith the control knob, state 314. Since clock and date adjustments occurrelatively rarely and not as a part of day-to-day patient care, thisstate is intentionally difficult to reach.

As indicated, based upon these state diagrams, software coding is withinthe skill of those in the art. Manipulation of the control knob toselect and/or display on-screen icons or numbers is a well knownsoftware technique. The interpretation of switch movements in variousstates also is readily accomplished. The presently preferredmicroprocessor programming for the interface is presented in Appendix B,which is attached hereto and made a part hereof.

Electrical noise and power interruptions are of concern in a ventilatorsystem that is supplying aeration to a patient. Most hospitals havebackup power that is activated shortly after a power outage, but thatpower is initially so irregular that even short interruptions mayinterfere with monitoring and control circuitry. A preferred powersupply 400 for the present invention is depicted in FIG. 11. The powerfor the ventilator 20 is taken from an available 120 volt, 60 Hz wallreceptacle through a plug 402. An electromagnetic suppression filter 404is applied across the active leads.

The 120 volt power is passed through a power transformer 406 andsupplied to a battery charger 408. The battery charger 408 charges aninternal 12 volt battery 410, and, optionally, an external 12 voltbattery 412. The ventilator power requirements, including theelectronics, are supplied as 12 volt DC current. This arrangement avoidsthe great part of interference on the power lines, as well as provides abackup power supply that permits the ventilator to run without anyexternal power for at least several minutes until another stable sourceof AC power is established.

The present invention thus provides an important advance in the art ofhuman lung ventilator systems, both in capability and ease of use.Although particular embodiments of the invention have been described indetail for purposes of illustration, various modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited except as by theappended claims.

What is claimed is:
 1. A ventilator system for use in assisting a patient to breathe, comprising:gas control means for mixing and regulating the gas that is to flow to a patient and receiving the gas that flows from the patient, the gas control means including flow control means for mixing together controllable amounts of two gases to form a mixed gas that flows to the patient, the flow control means including a separate, independently controllable custom-calibrated rotary flow control valve for each of the two gases; and controller means for independently controlling the two rotary flow control valves, wherein the controller means includesa calibration table for each of the rotary flow control valves, the calibration table including a correlation of gas flow rate as a function of rotary flow control valve position relative to a rotary flow control valve zero flow position, and means for automatically and periodically determining the rotary flow control valve zero flow position corresponding to a zero flow of each of the rotary flow control valves.
 2. The ventilator system of claim 1, wherein each of the rotary flow control valves includesa plate having an opening therethrough, a movable disk having a cam-shaped surface, the disk being disposed in proximity to the plate so that the cam-shaped surface covers a controllable portion of the opening in the plate as the disk is rotated, and a motor that rotatably and controllably drives the movable disk.
 3. The ventilator system of claim 2, wherein the controller means includesmeans for periodically closing the rotary valve to a fully closed position to establish a zero rotation position.
 4. A ventilator system for use in assisting a patient to breathe, comprising:gas control means for mixing and regulating the gas that is to flow to a patient and receiving the gas that flows from the patient, the gas control means includingflow control means for mixing together controllable amounts of two gases to form a mixed gas that flows to the patient, the flow control means including a separate, independently controllable custom-calibrated rotary flow control valve for each of the two gases, ventilation tube means extending to the patient for conducting the mixed gas to the patient and for receiving exhaled gas from the patient, and positive end expiration pressure means for controllably maintaining a minimum pressure in the exhaled gas; and controller means for independently controlling the two rotary flow control valves, wherein the controller means includes a calibration table for each of the rotary flow control valves, the calibration table including a correlation of gas flow rate as a function of rotary flow control valve position relative to a rotary flow control valve zero flow position, and means for automatically and periodically determining the rotary flow control valve zero flow position corresponding to a zero flow of each of the rotary flow control valves.
 5. The ventilator system of claim 4, wherein each of the rotary valves is driven by a stepper motor.
 6. The ventilator system of claim 4, wherein the gas control means further includes an inhalation flow sensor having a screen through which the mixed gas flows and a pressure sensor that senses the pressure drop across the screen, whereby the mixed gas flow rate is determined with a preexisting calibration relationship.
 7. The ventilator system of claim 4, wherein the gas control means further includes a nebulizer that injects a controllable flow of a third component into the mixed gas stream.
 8. The ventilator system of claim 4, wherein the ventilation tube means includes a filter for the mixed gas, a humidifier for the mixed gas, a water trap for the exhaled gas, and a heated filter for the exhaled gas.
 9. The ventilator system of claim 4, further includingcompressor means for supplying compressed gas to the gas control means, in the event that an external supply of compressed gas is not available.
 10. The ventilator system of claim 9, wherein the compressor means includes a compressor that operates continuously against a compressor accumulator and is sized to meet average rather than peak gas flow volume.
 11. The ventilator system of claim 4, wherein the controller means includes an operator interface having a cathode ray tube, and wherein the controller means further includes means for displaying to an operator patient and ventilator parameters, and operator control options on the cathode ray tube.
 12. The ventilator system of claim 4, wherein the controller means includes an operator interface having a single control knob for selecting ventilator control modes and parameter settings. 